In the past, variable reluctance motors have not been used in high accuracy servo systems which require fast settling time and precision positioning due to high force ripple and other nonlinearities. One example of such a variable reluctance motor is a coupled linear three phase variable reluctance motor 100 shown in FIG. 1. With reference to FIG. 1, the coupled linear three phase variable reluctance motor 100 comprises a stator 103 and an armature 101A and 101B including three phases A, B, and C. Each phase includes a coil which applies a current to the phase, thereby generating a magnetic field which causes the armature 101A and 101B to move with respect to the stator 103. A controller (not shown) provides a desired current at a selected time to each phase of the armature 101A and 101B to produce a desired force output. One or more sensors 104 are provided to detect the position of the armature 101A and 101B in relation to the stator 103.
A problem associated with known variable reluctance motor controllers is a relatively high force ripple, or variation in the force output, that results from the interaction of the armature and stator of the motor. For example, the force in the axis of motion with respect to the current varies non-linearly with respect to the relative position of the armature to the stator. As the relative position between the stator and armature changes, the tooth overlap position changes, and thus the relative reluctance path changes. As a result, the flux distribution between the three legs of the armature changes, and the amount of flux available at a given current as well as the percentage of the flux contributing to a flux in the linear direction changes. The force in the axis of motion also varies nonlinearly with respect to the magnitude of the current due to saturation effects. These types of localized nonlinear variations result from changes in the resultant force vector and the magnetic saturation characteristics of the motor. As a result of these nonlinear variations, the motor does not provide a linear force output with respect to phase current and position.
In order to maintain a low linear force tipple throughout an entire range of force levels of a variable reluctance linear motor and thereby achieve a relatively constant force output, a highly complex commutation method is needed to compensate for these variations. Further, the commutation method must also compensate for localized magnetic saturation effects due to air gap and current to control the force ripple of the motor.
One possible commutation method which may be used to control the magnitude of the force ripple includes the use of a table in which current corrections for all possible behaviors of the motor are stored. Based on the position of the motor components as sensed by the sensor(s) and the force output desired, a required phase current value is located in the table for each phase, and a corresponding current is provided to the phases of the motor. However, such a table requires a large amount of fast memory and is therefore expensive to implement.
A second possible commutation method includes the use of a complex algorithm for calculating the necessary phase currents based on empirical data obtained from the motor. However, the computer processing power necessary to perform the complex calculations in real time needed to control the force ripple requires a highly complex processor and/or a sacrifice in bandwidth due to the computation time required.